1. Field of the Invention
The invention relates to the domain of laser cutting, particularly in view of dismantling and/or disassembly operations.
2. Discussion of the Background
The advantage of using laser techniques for carrying out dismantling operations is that it provides some process flexibility (possibility of remote cutting and variation of the cutting distance), and large potential benefits in terms of secondary cutting waste (less aerosols, less swarf, smaller cutting widths).
However, very few of these systems have been used, for three main reasons:
the most powerful laser sources, particularly CO.sub.2 sources, require beam transport by mirrors since the wave length of CO.sub.2 sources is incompatible with materials used for power optic fibers. These mirrors enable the necessary movements for cutting. The resulting mechanism is very complicated, particularly when it is necessary to cross confinements and biological shielding in nuclear installations. PA1 the power of sources producing a beam that can be transported by optical fibers (for example Nd:YAG) did not exceed one kW until the last few years. Furthermore, the use of the optical fiber transmission technique causes problems which will be described below, PA1 traditional laser cutting processes require the use of assistance gases to make cutting feasible (flushing of molten material, protection of optics), which consequently requires that the part to be cut should be followed very closely (at about 1 to 2 mm). PA1 a Fourier diffractive element that is capable of separating an incident beam into n beams along n directions symmetric about an optical axis of the device, PA1 a diffractive element comprising n Fresnel lenses, capable of refocusing the n beams on the optical axis.
These constraints have made it very difficult to use lasers in cutting for dismantling for which confinement is compulsory, it is often necessary to pass through biological shielding, and proximity following is practically impossible since the geometry of objects to be cut is complex, not well known (since it is often difficult to measure, particularly in a nuclear environment) and very variable.
The use of optical fibers has also created a number of problems.
The emerging beam is disturbed due to its path, particularly at the exit from the fiber; for example, a beam with a Gaussian distribution about its center line has an annular shape at the exit from a conventional step-index fiber. Consequently, the maximum energy distribution is eccentric, and cutting performances are reduced.
For focal length of the order of one meter, beam disturbances can be corrected using conventional glass lenses, for example type BK7. These lenses are transparent to infrared (wave length 1.06 .mu.m) emitted by an Nd:YAG laser. But the weight of a device based on this technology and for this focal length is around 15 to 20 kg. Furthermore, this type of assembly is as delicate as a telescope with the same aperture (20 cm) and, for example, must be protected from shocks that would modify the settings and affect operational safety.
Consequently, remote servocontrol of the focal length (zoom) necessary for cutting parts with complex geometry is practically impossible using conventional glass mirrors or lenses with reasonable size, fragility and weight parameters. Therefore, it is not really feasible to achieve focal lengths exceeding a meter, for laser cutting for dismantling using conventional optical systems that are more suitable for use in the laboratory and/or for short focal lengths (less than one meter) than for cutting on nuclear sites at long distances.
The use of mirrors to manufacture focusing lenses would further increase the size for an equivalent weight and fragility, taking account particularly of stiffening devices and mirror mountings.